Poultry meat and eggs are important food sources, whose consumption increases continually due to the growth of the human population and their great quality-price ratio. The recent epidemic of avian influenza focused the public opinion on poultry health as well as food safety and security. Poultry vaccine technology became a worldwide concern.
Viral vectors expressing pathogen proteins are commonly used as poultry vaccines against targeted pathogens. Vaccines including such viral vectors induce expression of foreign pathogen proteins within infected cells, and thereby induce corresponding T-cell immunity.
It is well known that all herpes viruses, including herpes virus of turkey (HVT) and Marek's disease virus (MDV), can permanently survive in the body of an infected animal in a state of latent or persistent infection. Consequently, recombinant herpes viruses, in which a foreign gene derived from a pathogen has been integrated, have been developed to be used as viral-vectored vaccines increasing the duration of immunity of an immunized animal.
The genomic structure of HVT, its widespread usage as a vaccine against MDV and its ability to remain persistent in chickens make this virus an attractive vector for producing recombinant poultry vaccines.
Vaccine preparations have been developed to achieve effective avian vaccinations, using recombinant herpes viruses which incorporate a gene encoding a foreign antigen. Such vaccine preparations allow vaccination against both MDV (the vector) and another avian disease, through the inserted foreign DNA sequence.
Although such vaccine preparations provide efficient results of vaccination of avian species against many fatal diseases, competition and immunosuppression between pathogens can occur when birds are injected with two or more recombinant herpes viruses, each harboring a different foreign antigen gene.
Therefore, multivalent recombinant herpes viruses (i.e., harboring at least two different antigen genes) for immunizing simultaneously against different diseases would be particularly studied. However, up to now, recombinant HVTs (rHVTs) expressing multiple foreign genes turned out to be unstable, and all or part of the foreign genes are deleted during repeated passaging in culture cells. Accordingly, such unstable multivalent virus vectors cannot be used as efficient vaccines.
Accordingly, there is a need for stable multivalent recombinant viral vectors, allowing the co-expression of the foreign genes in infected cells.